A collagen gel is generally used as a scaffold (scaffold for cells) to be used in research and actual therapy in the regenerative medicine field. However, the collagen gel is derived from an animal and hence may cause an unknown infectious disease. As means for eliminating the concern about the unknown infectious disease, there exists a scaffold derived from a chemically synthesized material. Examples of such material include a self-assembling peptide disclosed in Patent Document 1 or Patent Document 2. However, a scaffold (peptide gel) formed of the self-assembling peptide of Patent Document 1 or 2 is insufficient in mechanical strength and hence involves such a problem in handleability that the scaffold may collapse when grasped with tweezers, for example. Further, the self-assembling peptide gel of Patent Document 1 involves such a problem that the gel is insufficient in transparency at neutral pH environment.